Those cosigners include some who have an interest by being trans researchers in other domains (such as software engineering, bioinformatics, or ENT surgery), others have a rich set of domain experience (e.g., people whose direct area of research is trans youth helathcare or clinicians for the same). They cover multiple continents and institutions.
Many (if not all) of those involved are themselves trans or at least LGBTQ. These are people with a stake in the game, so to speak.